Even though the proposed deponents may have been unavailable at trial, courts have routinely denied Rule 15 deposition requests if the proposed witness testimony was not material. Dr. Lorandos notes that neither the Rule nor the Advisory Committee’s note defines materiality, and courts have assessed this prong of the “exceptional circumstances” test in a variety of ways, asking, for example, whether the testimony is essential or critical to the defense, exculpatory, or capable of negating an element of the government’s case, or instead is cumulative or merely corroborative.